Over Propping Questions

[Nomad Willy]

We talk about this frequently but this is becoming one of the better discussions on this issue IMO.
And we're not even off topic.

 

[River Cruiser]

IMHO over propping is never a good thing. The amount of fuel being injected isn't determined by the throttle position. The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load. As the load increases the governor opens the rack to increase fuel, the governor on boat in question is delivering full fuel at a much reduced rpm. It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it.

 

[sunchaser]

RC

I may be misreading what you said, but I'm not sure there is a direct correlation between RPM and fuel delivery. Even with a perfectly propped vessel. RPM and fuel consumption will independently vary dependent upon boat load, sea state and wind velocity.

 

[stanfromhell]

Thanks eveyone for your well thought out and very informative comments, I truly appreciate your responses.
I contacted Carver and the props on the boat are the same diameter and pitch as when the boat left the factory.
So......Props are correct, hull is clean, injectors have recently been rebuilt, no black smoke when running at 2100 RPM, engine hours are low, both exhibit identical sympoms and neither has excessive blowby.

Perhaps the wrong nozzles were installed when the injectors were rebuilt, but I'm not going to take a chance where the wrong decision could cost many, many thousands of dollars somewhere down the road.

My search for the perfect boat continues.

 

[CaptSteve53]

Moving backwards to my original mundane post-Have you checked the throttle is moving to the full throttle stops on the injection pumps?

Is there enough air getting to the engines?

Just saying!!

 

[Cigatoo]

“I contacted Carver and the props on the boat are the same diameter and pitch as when the boat left the factory.“

There lies your problem. That boat was propped to perform correctly as it left the factory. Now you have thousands of pounds added to the weight of the boat and therefore the props have to be adjusted to compensate for the extra weight / load

Go back to basics as Capt Steve mentioned above. Or get a qualified diesel guy to look at it. YOU DO NOT WANT TO BE OVERPROPPED.

 

[psneeld]

Some tips from a marine engineer who works with diesels every day.....

"The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load."

Unless his recreational trawler has been retrofitted with a torque sensitive governor (in which case this whole discussion is moot) his governor only senses and responds to rpm changes either side of the setpoint determined by the "throttle" lever.

"As the load increases the governor opens the rack to increase fuel ..."

No, as the rpm decreases below setpoint the governor increases fuel flow.

" ...the governor on boat in question is delivering full fuel at a much reduced rpm."

The only time the governor would deliver full fuel flow is when it can no longer increase rpm to the setpoint. At that time the governor should limit fuel as the internal delivery control reaches the "fuel stop." It would take a seriously "over propped" condition to have this occur at any rpm setting below maximum.

"It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it."

The only time the governor would "over fuel" is when the load on the engine exceeds that which the engine can produce at a given rpm. This can be determined by the engine power curve. The propeller can be grossly over pitched or oversized but until the torque required to turn it exceeds what the engine can produce at a given rpm it will not present an overload condition.

 

[Cigatoo]

Some tips from a marine engineer who works with diesels every day.....

"The governor senses the load delivers the amount of fuel necessary to maintain the rpm at that load."

Unless his recreational trawler has been retrofitted with a torque sensitive governor (in which case this whole discussion is moot) his governor only senses and responds to rpm changes either side of the setpoint determined by the "throttle" lever.

"As the load increases the governor opens the rack to increase fuel ..."

No, as the rpm decreases below setpoint the governor increases fuel flow.

" ...the governor on boat in question is delivering full fuel at a much reduced rpm."

The only time the governor would deliver full fuel flow is when it can no longer increase rpm to the setpoint. At that time the governor should limit fuel as the internal delivery control reaches the "fuel stop." It would take a seriously "over propped" condition to have this occur at any rpm setting below maximum.

"It is always over fueling all the way through its rpm range and it's been that way as long as the props have been on it."

The only time the governor would "over fuel" is when the load on the engine exceeds that which the engine can produce at a given rpm. This can be determined by the engine power curve. The propeller can be grossly over pitched or oversized but until the torque required to turn it exceeds what the engine can produce at a given rpm it will not present an overload condition.

Exactly correct. The boat is overpropped.

 

[Cigatoo]

I would also be willing to bet that if you were to add EGT gauges you would see that the EGT is much higher then what the mfg is calling for. The injection system is dumping fuel into the engine, trying its best to regain the requested RPM. All that extra fuel is just creating heat that does damage to the engine.

Those props were set at the factory to help sell boats. They want to highlight maximum speed and fuel efficiency. As soon as you start adding weight you are asking for more HP and torque than the power plant can produce. You have to either decrease the load (remove weight) or change the torque requirements via decreasing the pitch on the props.

 

[sunchaser]

PS
Thanks for post 37 and the imbued knowledge from a solid marine engineer on how diesel governors work.

When cruising offshore and climbing large swells, the momentary increased fuel consumption and harder working engine at the same RPM, are clearly evident.

 

[stanfromhell]

I could believe that over propping is the cause if both engines ran 100 or so RPM below the governed speed. But these engines are shy by 500 RPM with a clean bottom, fuel tanks half full, water and holding tanks nearly empty and a 10' inflatable topsides. Carver engineers would not be dumb enough to OK that situation no matter what their marketing group wanted. Engine speed and fueling is controlled by ECM's and one is quite new. During sea trials the Volvo mechanic ran a heat gun all over the exhaust ,coolers, heads etc and found nothing alarming. It's a nice boat but I'n not going to buy it.

 

[Baker]

I could believe that over propping is the cause if both engines ran 100 or so RPM below the governed speed. But these engines are shy by 500 RPM with a clean bottom, fuel tanks half full, water and holding tanks nearly empty and a 10' inflatable topsides. Carver engineers would not be dumb enough to OK that situation no matter what their marketing group wanted. Engine speed and fueling is controlled by ECM's and one is quite new. During sea trials the Volvo mechanic ran a heat gun all over the exhaust ,coolers, heads etc and found nothing alarming. It's a nice boat but I'n not going to buy it.

PLEASE PLEASE listen to what these people are saying. The Carver engineers do as they are told. My Carver left the factory with 23x27 props. I now have 23x25....IOW, lower pitch. If my props were "as they left the factory" I would be well overpropped. I am perfectly propped right now....about 100RPM over rated RPM. Carver is a large company. And all, and I mean ALL, large companies are driven by their marketing department. That marketing department wants to be able to post about a certain performance. That performance is gained when the boat is light and empty. Boats gain weight over time. That weight is in the MANY thousands of pounds. Also a fouled prop and a fouled hull. I will reiterate....LISTEN TO WHAT THESE PEOPLE ARE SAYING.

There used to be a guy on here that would not listen when the same argument was being made. He had the Cummins Guru, Tony Athens, telling him exactly what we are telling you. And his response is exactly your response. And guess what, he was replacing an engine within a year. Do not go chasing these ghosts down other rabbit holes. We have defined the rabbit and we have defined the hole.

The last 10-15 posts are excellent posts...especially PSNeeld's. When you set your "throttles" you are setting spring pressure on the governor. Flyweights help maintain that spring pressure. When the engine slows down, it is sensed by those flyweights as they "spring in" and that is connected to the rack which adds fuel to get the spring pressure and flyweights back in balance. That is basically how a mechanical diesel governor works....just to clarify terms being used like "requested rpm" and "sensed RPM". In a mechanical governor, it is all....wait for it....mechanical. No computers.

 

[Baker]

I missed the part about you not buying the boat. Sorry about my overly dramatic post...������. But maybe someone will learn something. Overpropping a planing boat is never good.

 

[River Cruiser]

RC



I may be misreading what you said, but I'm not sure there is a direct correlation between RPM and fuel delivery. Even with a perfectly propped vessel. RPM and fuel consumption will independently vary dependent upon boat load, sea state and wind velocity.

You are correct, the amount of fuel delivered is controlled by the governor reacting to the load being applied. As the load increases fuel is increased, as the load decreases the fuel will decrease to maintain the set rpm. Being overloaded the fuel delivered and burned will increase combustion chamber and exhaust gas temperatures, not a good thing for long engine life. The problem when over propped goes all the way through the rpm range meaning more fuel is delivered at any rpm setting than would be required if correctly propped or loaded.

 

[psneeld]

More words of wisdom from the marine engineer who covered diesel governing.

He responded to this sentence....

"“Being overloaded the fuel delivered and burned will increase combustion chamber and exhaust gas temperatures, not a good thing for long engine life. “

With this response.....

"The sky is not falling. Any increase in fuel flow will increase the exhaust temperature, that is why we monitor EGT, it is a good method to determine engine load. Stating that increased combustion chamber and exhaust gas temperatures are not a “good thing” without a whole lot of qualifying conditions is pure chicken little stuff.

Then this statement....

“The problem when over propped goes all the way through the rpm range meaning more fuel is delivered at any rpm setting than would be required if correctly propped or loaded.”

His thoughts.......

"The “problem” does not exist until the engine climbs above the maximum allowable EGT and that will only occur when the engine is asked to produce more power than it is capable of delivering to a propeller that is capable of absorbing much more.

It appears to me that the issue here is based on people looking at the rpm obtainable with a propeller that absorbs maximum power a little below the high idle speed. They seem to ignore the fact that their engine will run quite nicely at less than full power with a propeller that moves the boat further through the water at a given rpm. As long as that propeller does not absorb more than the power available per the power curve at a given rpm there is no overload condition. All you have is a lower powered propulsion system that lacks the “turn down ratio” of the ideal propeller curve. You lose the ability to move through the water slowly at idle in gear and you lose top speed and acceleration because you have effectively downrated your engine. Unless you adjust the fuel limiter and the throttle stop you risk an overload condition when moving the throttle to the stop while attempting to regain the performance you threw away by “overpropping.”"

 

[Fletcher500]

Good discussion, and a reminder regarding changing weights over time on a vessel and prop pitch.

We had a new planing boat that was sea trialed by the engine rep, the Broker, and myself, before they would hand me the keys, and put the engines under warranty.

We couldn't make the required RPM at WOT so we all went home. New props shipped in, tried it again, and the engine service rep who is representing the mother ship who made the engine in this case signed it off. Keys given to me at that point, and the warranty clock started.

 

[Delfin]

When I purchased Delfin, she was an empty hull with just a new CAT and genset. The CAT dealer came down to the vessel to do a delivery inspection underway at full load. He measured EGT, max rpm, etc. and signed off. I can well see the Carver folks doing the same thing to a vessel leaving their factory, and while not as light as Delfin, the brand new Carver would be quite a bit lighter than when in use by the owner. If you think about it, my CAT guy could only certify the engine "as is", since he had no idea what the final vessel weight might be.

Long story short, after adding a few tons of cabinetry and furnishings, Delfin probably weighed 15 tons more than when an empty hull and we were "over propped." No harm done since we never powered beyond around 35% of rated power but after flattening the props a couple of inches the engine runs EGT around 125 degrees lower, she doesn't run at 4 knots at idle and she seems "sweeter" at slightly higher rpm with no change I can measure in fuel consumption.

Point is, factory prop settings can easily be too aggressive since they match an empty boat. Whether they hurt anything depends on how the vessel has been run, but I would be very uncomfortable buying a planing hull that has run more than a few hours at WOT but reduced design rpms. Rhw OP is wise to pass, IMO.